Users of mobile communication devices, such as cellular telephones, Personal Digital Assistants (PDAs) or laptop computers that include wireless capability, etc., often experience performance problems, such as dropped calls, poor call quality, and an inability to connect with the network. Such problems are often the result of interference from other wireless signals in the area. Additionally, however, such problems can also be the result of what is called multi-path interference.
The term multi-path is a term that describes how a signal transmitted in a wireless environment travels along multiple paths from the transmission source to the destination or receiver. For example, when a base station transmits a signal to a mobile communication device, the energy comprising the signal spreads out. Some of the energy can travel along a direct line to the mobile communication device. This direct line is one path. Some of the energy can, e.g., reflect off a building and then reach the mobile communication device. The reflected signal path being a second path. Similarly, some of the energy can reflect off other buildings, or mountains, etc., before reaching the mobile communication device. The different paths traveled by the signal energy from the base station to the mobile device are referred to as multi-paths, and the associated signal energies are referred to as the multi-path signals, or sometimes multi-path for short.
The multi-path signals combine with each other in the mobile communication device receiver. At times the multi-path signals will combine constructively, but at other times the signals will combine destructively, i.e., the signals will combine in such a manner that they at least partially cancel each other out, or interfere with each other. This is because the multi-path signals can be out of phase with each other due to the different lengths of the paths traveled. Destructive multi-path combining, or interference, can lower the signal-to-noise ratio in the receiver, and affect other signal parameters, causing the problems referred to above. Such destructive multi-path interference is often referred to as fading, i.e., it causes the signal as seen by the mobile communication device receiver to fade out.
Another source of interference in communication systems is jamming transmissions. These transmissions, or the source of these transmissions, can also be referred to as jammers. Jamming is sometimes defined as deliberate interference in communications by means of unwanted signals that are intended to render unintelligible or to falsify the whole or part of the desired signal. As used herein, however, jamming can include interference that is not intentional, for example transmissions from other mobile communication devices.
Spatial diversity has been used to combat the problem of destructive multi-path interference, fading, jamming, etc. In its simplest form, spatial diversity comprises two antennas spaced a certain distance apart. Often, the distance between the antennas is related to the wavelength of the signal being received, e.g., a half wave length or quarter wavelength of separation. The idea of spatial diversity is that if one antenna at one location is experiencing, e.g., fading due to the destructive combining of multi-path signals, then it is likely that the other antenna will not be experiencing fading. When the first antenna is experiencing fading, the second antenna can be selected. Thus, spatial diversity can improve performance and help overcome, e.g., the problems referred to above. Moreover, spatial diversity can extend to any number of antennas.
Stated another way, the signal received by the second antenna is uncorrelated with the signal received by the first. If the antennas were receiving the same signal, then the signals produced by the antennas would be highly correlated. Obviously, having two antennas receiving highly correlated signals is not necessarily helpful, since problems with the received signal for open of the antennas will also be experienced by the other antenna. Thus, an aim of including spatial diversity is to add uncorrelated signals that can be used to improve receiver performance.
Polar diversity can also be implemented in order to improve performance in a mobile communication device. It will be understood that the polarity, or polarization of an antenna affects how the antenna receives signals. Thus, an antenna that has a different polarity relative to another antenna can provide uncorrelated signals relative to the other antenna even if the antennas are co-located.
Antennas can have different radiation patterns as well, which will affect how the antenna sends and receives signals. Diversity with respect to the radiation pattern of two antennas can also provide uncorrelated signals that can be used to improve reception.
Another form of diversity that is commonly used in conventional mobile communication devices is time diversity. For example, many conventional mobile communication devices include a rake receiver that can differentiate different multi-paths. The rake receiver is actually a combination of sub-receivers delayed with respect to one another.
In certain conventional communication devices, a plurality of spatially and or polar diverse antennas are incorporated into the communication device. The device can then be configured to switch between different antennas and/or different polarities in order to attempt to improve the received signal quality. This type of configuration is referred to as a smart antenna configuration. When the received signal quality in a device with a smart antenna is degraded beyond a certain point, the device can switch between a plurality of antenna configurations in an attempt to improve the received signal quality.
A drawback to such conventional approaches, however, is that only one antenna configuration is used at a given time. As a result, alternate antenna configurations that may be receiving signals with better signal quality that could aid in overall reception will not be used.